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Cardiovasc Eng
DOI 10.1007/s10558-010-9113-0
ORIGINAL PAPER
Afterload Assessment With or Without Central Venous Pressure:
A Preliminary Clinical Comparison
Glen Atlas • Jay Berger • Sunil Dhar
Ó Springer Science+Business Media, LLC 2010
Abstract A clinical comparison, of two methods of
afterload assessment, has been made. The first method,
systemic vascular resistance index (SVRi), is based upon
the traditional formula for afterload which utilizes central
venous pressure (CVP), as well as cardiac index (Ci), and
mean arterial blood pressure (MAP). The second method,
total systemic vascular resistance index (TSVRi), also uses
MAP and Ci. However, TSVRi ignores the contribution of
CVP. This preliminary examination, of 10 randomlyselected ICU patients, has shown a high degree of correlation (ranging from 90 to 100%) between SVRi and
TSVRi (P \ 0.0001). Furthermore, there was also a high
degree of correlation (ranging from 94 to 100%) noted
between the hour-to-hour change in SVRi with the hour-tohour change in TSVRi (P \ 0.0001). The results, of this
pilot study, support the premise that the use of CVP may
not always be necessary for afterload evaluation in the
clinical setting. Minimally-invasive means of measuring
both Ci and MAP, without CVP, may be adequate for use
in assessing afterload.
Keywords Systemic vascular resistance index Total
systemic vascular resistance index Central venous
pressure Pulmonary artery occlusion catheter
Introduction
Examining SVRi
Clinicians have typically used systemic vascular resistance
index (SVRi) as a measure of afterload in the management
of critically ill patients (Melo and Peters 1999) as well as
those undergoing extensive cardiac, vascular, and thoracic
surgical procedures (Barash et al. 2009). SVRi is defined as
(Li 2000, 2004):
SVRi ¼
G. Atlas (&) J. Berger
Department of Anesthesiology, University of Medicine
and Dentistry of NJ, Newark, NJ, USA
e-mail: [email protected]
G. Atlas
Department of Chemistry, Chemical Biology, and Biomedical
Engineering, Stevens Institute of Technology, Hoboken, NJ,
USA
S. Dhar
Department of Mathematical Sciences, New Jersey Institute
of Technology, Newark, NJ, USA
S. Dhar
Center for Applied Mathematics and Statistics, New Jersey
Institute of Technology, Newark, NJ, USA
ðMAP CVPÞ
80
Ci
ð1Þ
where MAP is mean arterial pressure, CVP is central
venous pressure, and Ci is cardiac index. Note that the
constant 80 allows for SVRi to be expressed with the
dimensions of: dyne s cm-5 m-2 or ‘‘resistance units’’. A
list of abbreviations, dimensions, and associated terminology is shown in Table 1.
Traditionally, the determination of SVRi has required
invasive hemodynamic monitoring with a pulmonary artery
occlusion catheter (PAOC) to provide measurements of
both Ci and CVP. Note that MAP can be obtained noninvasively, with a blood pressure cuff, or in a minimallyinvasive manner with a peripherally-placed arterial
catheter.1
1
Typically, the radial artery is used for this purpose.
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Cardiovasc Eng
Table 1 List of abbreviations,
dimensions, and terminology
a
Based upon patient height
b
Based upon patient body
surface area
Abbreviation
Meaning
Dimensions
BMi
Body mass indexa
kg m-2
CAD
Coronary artery disease
Ci
Cardiac indexb
CVP
Central venous pressure
mmHg
DBP
Diastolic blood pressure
mmHg
DM
Diabetes mellitus
l min-1 m-2
EDM
Esophageal Doppler monitor
EF
Ejection fraction
HTN
Hypertension
MAP
Mean arterial pressure
OSA
Obstructive sleep apnea
%
mmHg
PAOC
Pulmonary artery occlusion catheter
SBP
Systolic blood pressure
SVRi
Systemic vascular resistance indexb
TSVRi
mmHg
Total systemic vascular resistance index
The clinical utility of PAOCs has been questioned as
outcome studies have generally not supported their use
(ASA 2003; Sandham et al. 2003; NHLBI 2006; Warzawski
et al. 2003). Furthermore, minimally-invasive and noninvasive means of measuring Ci have emerged. These
technologies include the esophageal Doppler monitor
(EDM) or CardioQÒ (Deltex Medical, UK) and the FloTracÒ (Edwards Lifesciences, USA).2 Note that the EDM
determines Ci using the velocity of blood flow in the distal
aorta (Dark and Singer 2004). Whereas the FlowTracÒ
requires a peripheral arterial catheter to measure pulse
pressure and uses a ‘‘pulse contour analysis’’ method to
estimate cardiac output (Manecke 2005). The assumptions
made with pulse contour analysis may be erroneous as
clinical comparisons, of this device with the EDM, have
demonstrated significant inconsistencies in stroke volume
determination (Chatti et al. 2009).
Measurement of the change in electrical impedance or
‘‘bioimpedance’’ of the thorax, or whole-body, as a function of a time, has also been developed to non-invasively
assess Ci (Barin et al. 2000; Ouzounian et al. 1996).
Unfortunately, this method has also demonstrated inconsistent correlation with traditional methods of measuring
cardiac output (Imhoff et al. 2000; Simon et al. 2009).
None of these minimally-invasive techniques, of Ci
evaluation, allow for CVP measurement. An ideal method
of afterload assessment would therefore also be minimally
invasive and allow for continuous measurements. It should
be noted that PAOCs only allow for determination of SVRi
on an intermittent basis.
dyne s cm-5 m-2 (resistance units)
b
dyne s cm-5 m-2 (resistance units)
Examining TSVRi
Another method of afterload assessment can be defined as
total systemic vascular resistance (TSVRi) in which the
contribution of CVP is ignored (Wang et al. 2008; Atlas
2008):
TSVRi ¼
MAP
80
Ci
Thus, the purpose of this study was to assess the
correlation of SVRi to TSVRi using the data acquired from
PAOCs. Therefore, the contribution of CVP, to afterload
assessment, could be made. In addition, the hour-to-hour
changes in both SVRi and TSVRi were also examined. This
was done to determine how changes in CVP effect changes
in afterload.
Ultimately, the use of minimally-invasive and noninvasive devices could possibly be utilized, for the
assessment of afterload, without the potential complications associated with pulmonary artery cannulation.
These complications are summarized in Table 2 (ASA
2003).
Differential Analysis
A differential examination of both SVRi and TSVRi was
also made to assess the hour-to-hour changes in these
values. It should be noted that clinicians frequently
examine the ‘‘trend’’ in afterload over time. Thus, the hourto-hour changes in SVRi and TSVRi are defined as:
DSVRi ðkÞ ¼ SVRi ðk þ 1Þ SVRi ðkÞ;
k ¼ 1; 2. . .ðN 1Þ
2
Atys Medical (France) also manufactures an EDM. However, it is
not currently available in the USA.
123
ð2Þ
and
ð3Þ
Cardiovasc Eng
Table 2 List of complications associated with PAOCs (ASA 2003)
Complication
Incidence in most
studies (%)
Arterial puncture
B3.6
0.3–1.9
Minor dysrthymias
[20
Severe dysrthymias (ventricular
tachycardia or fibrillation)
0.3–3.8
Pulmonary artery rupture
0.03–0.7
Positive catheter-tip cultures
C19
Catheter-related sepsis
0.7–3.0
Venous thrombosis
0.5–3.0
Pulmonary infarction
0.1–2.6
Valvular/endocardial vegetations or endocarditis
2.2–7.1
DTSVRi ðkÞ ¼ TSVRi ðk þ 1Þ TSVRi ðkÞ;
k ¼ 1; 2. . .ðN 1Þ
MAP =
ð4Þ
Note that k ranges from the first hour to the hour preceding
the final hour (N - 1). Where N represents the final hour.
The mathematical relationship, between DSVRi and
DTSVRi, using both an algebraic method as well as the
definition of the total differential, is documented in the
Appendix.
2
1
DBP + SBP
3
3
ð5Þ
As this was a preliminary or ‘‘pilot’’ study, no attempt to
stratify data, based upon patient acuity, severity, or
survival, was made. Furthermore, the hemodynamic
influence, of any of each patient’s medications, such as
sedatives or vasoactive medications, was also not included.
SVRi and TSVRi were calculated, for each hour, using
Eqs. 1 and 2 respectively. Similarly, DSVRi and DTSVRi
were also calculated, on an hourly basis, using Eqs. 3 and 4.
3000.0
Resistance Units
(dyne·s·cm-5·m-2)
Pneumothorax
Care Unit (CTICU) at University Hospital/Newark. Note
that this study was strictly a retrospective chart review of
preexisting data. Each patient’s demographic information
is shown in Table 3.
Hemodynamic data, from all 10 patients, were initially
obtained each hour for at least 24 h. These data included:
Ci, CVP, and MAP. Note that MAP was determined from
either an indwelling arterial catheter or a non-invasive
blood pressure cuff. For the purposes of this study, MAP
was calculated using (Li 2000, 2004):
Methods
Data Acquisition and Analysis
SVRi and TSVR i
Patient #1
2500.0
2000.0
1500.0
1000.0
SVRi
TSVRi
500.0
1
After institutional review board approval, the medical
records of 10 randomly-selected patients, whose treatment
had included indwelling PAOCs, were examined. All
patients had been admitted to the Cardiothoracic Intensive
3
5
7
9 11 13 15 17 19 21 23 25
Time
(hours)
Fig. 1 SVRi and TSVRi calculated, on an hourly basis, using the data
from patient #1
Table 3 Demographic data of the ten randomly-selected cardiothoracic ICU patients
HTN CAD DM Tobacco OSA EF
Patient Age
Gender Weight Height BMi
(kg m-2)
use
(years)
(kg)
(m)
Procedure
1
46
Male
116
1.85
33.64
?
Ascending aortic aneurysm
2
65
Male
124
1.80
38.07
?
3
64
Male
79
1.72
26.30
?
?
4
76
Male
73
1.70
25.06
?
?
5
72
Female
57
1.57
22.86
?
6
47
Female
94
1.65
34.61
?
7
53
Male
104
1.52
44.91
?
8
70
Male
106
1.72
35.58
?
9
62
Male
70
1.72
23.41
10
57
Female
80
1.75
25.99
?
?
?
50%
?
?
55–60% Esophagectomy
?
55%
Coronary artery bypass
graft(s)/mitral valve replacement
?
?
75%
Coronary artery bypass graft(s)
?
?
70%
Aortic valve replacement
?
?
?
?
25–30% Coronary artery bypass
graft(s)/mitral valve replacement
?
?
56–60% Mitral valve repair
?
50%
?
?
40–45% Coronary artery bypass graft(s)
Coronary artery bypass graft(s)
?
?
50%
Mitral valve replacement
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Cardiovasc Eng
Results
Fig. 2 A linear correlation, between SVRi and TSVRi, is illustrated
using the data from patient #1
Resistance Units
(dyne·s·cm-5·m-2)
ΔSVRi and ΔTSVRi
Patient #1
Based upon this preliminary study, SVRi and TSVRi are
closely correlated. This is exemplified in both Figs. 1 and 2
which graphically demonstrate the data from patient #1.
The correlation coefficients, from all ten patients’ data,
ranged from 90 to 100%. Furthermore, all correlation
coefficients were statistically significant (P \ 0.0001).
Similarly, DSVRi and DTSVRi, which represent the
hour-to-hour changes in SVRi and TSVRi, were also highly
correlated. These correlation coefficients ranged from 94 to
100% (P \ 0.0001). Figures 3 and 4, which are also based
upon the data from patient #1, illustrate this relationship.
All correlation coefficients and their associated 95%
confidence intervals are documented in Table 4.
Discussion
Time
(hours)
Fig. 3 Hour-to-hour changes in SVRi and TSVRi using the data from
patient #1. These changes are referred to as DSVRi and DTSVRi
respectively
Statistical analysis was accomplished, on a patient-bypatient basis, using a Pearson product-moment correlation
coefficient (Kutner et al. 2004; Kreyszig 1999). In addition, a
Fisher’s z-transformation was also used to determine statistical significance, and associated confidence intervals, from
the correlation coefficients derived from each patient’s data
(Hawkins 1989). Furthermore, it was assumed that all data
points were from a bivariate normal distribution.
The assessment of afterload remains an important aspect of
hemodynamic monitoring during anesthesia and critical
care situations. Specifically, clinical conditions such as:
septicemia, adrenal insufficiency, and pancreatitis frequently require afterload measurement (Melo and Peters
1999). In addition, cardiac, thoracic, and vascular surgical
procedures also may necessitate intraoperative and postoperative afterload assessment (Barash et al. 2009).
Furthermore, severely injured patients, particularly
those with neurogenic shock (Bilello et al. 2003) or with
systemic inflammatory response syndrome (SIRS) (Cremer
et al. 1996; Carrel et al. 2000; Kohsaka et al. 2005), may
require afterload measurements. Other conditions requiring
possible afterload assessment include: hypertensive crisis,
congestive heart failure, aortic dissection, hepatic transplantation, burn injury, and the management of pheochromocytoma (Barash et al. 2009).
Δ
Fig. 4 A linear correlation,
between DTSVRi and DSVRi, is
documented using the data from
patient #1
Δ
123
* P \ 0.0001
0.94* (0.88, 0.97)
0.99* (0.98, 1.00)
1.00* (0.99, 1.00)
0.99* (0.98, 1.00)
0.93* (0.87, 0.97)
0.97* (0.93, 0.98)
0.97* (0.93, 0.99)
0.96* (0.91, 0.98)
1.00* (0.99, 1.00)
1.00* (0.99, 1.00)
0.97* (0.94, 0.99)
0.97* (0.93, 0.99)
0.99* (0.97, 0.99)
0.99* (0.99, 1.00)
0.94* (0.85, 0.97)
0.95* (0.87, 0.98)
0.92* (0.83, 0.97)
0.90* (0.77, 0.95)
0.91* (0.79, 0.96)
TSVRi versus SVRi
DTSVRi versus DSVRi
0.96* (0.91, 0.98)
8
7
6
5
4
3
2
1
Patient
Table 4 The correlation coefficient, R (in bold), and the corresponding 95% confidence intervals for both TSVRi versus SVRi and DTSVRi versus DSVRi
9
10
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However, the use of PAOCs has been associated with
questionable benefit as well as significant morbidity, mortality, and economic cost (ASA 2003; Domino et al. 2004;
Stewart et al. 1998). In particular, the rising incidence of
antibiotic-resistant nosocomial infections may further
question, and limit, the use of these highly invasive hemodynamic monitors (Blot et al. 2005; Richards et al. 2000).
Minimally-invasive hemodynamic monitors, specifically
the EDM, have become well established as reliable, safe,
and accurate (Madan et al. 1999; DiCorte et al. 2000; Dark
and Singer 2004). Furthermore, the EDM can be placed
either orally, in intubated patients, or nasally, in awake
patients (Atlas and Mort 2001; Dodd 2002). Whereas the
FlowTracÒ requires a peripheral arterial cannula. It should
be noted that both of these instruments provide approximations of cardiac output. However, in clinical trials, the
EDM has been shown to provide reproducible and reliable
measurements of cardiac output (Singer et al. 1989; Valtier
et al. 1998). In addition, the EDM is extremely accurate in
measuring changes in cardiac output (DiCorte et al. 2000;
Dark and Singer 2004).
Therefore, the ability to assess afterload, with minimally-invasive devices, without CVP measurement, could
offer significant benefit in patient safety. This preliminary
study has shown that CVP appears to play a minor role in
afterload assessment. Furthermore, the hour-to-hour changes in afterload appear to be minimally affected by the
exclusion of CVP.
However, specific clinical conditions may nonetheless
require CVP measurement for management purposes.
These would include such pathological states as: superior
vena cava syndrome, congenital cardiac anomalies, intracardiac tumors, and cardiac tamponade. These conditions
can create significant increases in CVP by restricting
venous blood flow (Magder 2005).
Furthermore, jugular venous distention (JVD), which is
a diagnostic ‘‘hallmark’’ of CHF, is also associated with an
increase in central venous pressure (Drazner et al. 2001).
However, invasive monitoring of CVP is rarely necessary
for the clinical diagnosis and management of this common
condition (Senni et al. 1998).
Certain clinical situations may artificially alter CVP. The
use of positive end-expiratory pressure (PEEP), for patients
with severe pulmonary disease, is one example (Magder
2005). In addition, the simultaneous use of the central venous
cannula, for intravenous fluid administration during CVP
measurements, may produce erroneous data (Ho et al. 2005).
Furthermore, CVP may not be meaningful for patients who are
in the prone or lateral decubitus positions (Soliman et al.
1998). A malpostioned pressure transducer can also significantly change CVP measurements (Magder 2005). Therefore,
in these clinical situations, TSVRi may be advantageous over
SVRi due to the possible inaccuracy of CVP.
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Cardiovasc Eng
Continuous afterload assessment would also be useful in
‘‘highly critical’’ situations. These would possible include
patients on potent vasoactive medications for treatment
during shock or sepsis. Patients undergoing removal of a
pheochromocytoma may also benefit from this.
Lastly, other hemodynamic parameters and terminology,
associated with afterload assessment, exist. These include:
pulse wave velocity, impedance, and characteristic
impedance (Segers et al. 2007; Nichols and O’Rourke
2005; Milnor 1989). However, SVR and SVRi remain the
‘‘mainstays’’ of clinical afterload measurement within the
hospital-based clinical community.
CVP1 CVP2
Ci1
Ci2
CVP1 CVP2
thus:
0
Ci1
Ci2
ð4AÞ
Finally:
DSVRi DTSVRi
ð5AÞ
This concept can also be illustrated using the definition
of the total differential:
DSVRi ¼
oSVRi
oSVRi
oSVRi
DMAP þ
DCVP
DCi þ
oMAP
oCi
oCVP
ð6AÞ
and
DTSVRi ¼
oTSVRi
oTSVRi
DMAP þ
DCi
oMAP
oCi
ð7AÞ
Conclusions
This preliminary study has favorably compared, and
correlated, afterload assessment with CVP measurement
(SVRi) to afterload assessment without CVP measurement
(TSVRi). Further research, which would include prospective trials to assess the benefits, risks, and limitations
of TSVRi measurement, appear indicated. In addition,
these studies could be carried out with minimally-invasive
hemodynamic monitors. A clinical comparison, of various
means of afterload assessment, could then be implemented. Thus, a comparison of SVRi to TSVRi, with
respect to patient outcome, complications, and cost, could
be made.
Appendix
The Interrelationship Between DSVRi and DTSVRi
DSVRi is calculated using the definition of SVRi from
Eq. 1:
ðMAP2 CVP2 Þ
ðMAP1 CVP1 Þ
DSVRi ¼
80 80
Ci2
Ci1
ð1AÞ
Rearrangement yields:
MAP2 MAP1
CVP1 CVP2
DSVRi ¼
80 þ
80
Ci2
Ci1
Ci1
Ci2
ð2AÞ
Substituting the definition of TSVRi from Eq. 2 results in:
CVP1 CVP2
DSVRi ¼ DTSVRi þ
80
ð3AÞ
Ci1
Ci2
Realizing that:
123
oSVRi
oCVP DCVP oTSVRi
oSVRi
oMAP and oCi
Since
oSVRi
oMAP
0 then DSVRi DTSVRi . Also:
i
oTSVR
oCi .
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